Koster sent a letter to lawmakers warning that the bill contains “flawed public policy” that could prevent police from cooperating with federal authorities and allow criminals to sue police and prosecutors for referring gun cases to federal officials.

The Republican-led Legislature is to meet Sept. 11 to consider overriding Democratic Gov. Jay Nixon’s veto of the legislation. Several Democrats have said they are likely to vote with Republicans to enact House Bill 436.

If the veto is overridden, a quick lawsuit is expected.

Among other things, the bill declares invalid in Missouri any federal measures “which infringe on the people’s right to keep and bear arms” and allows state misdemeanor charges to be brought against federal authorities who attempt to enforce those laws. Charges also could be filed against anyone who publishes a gun owner’s name, address or other identifying information.

Nixon, a former state attorney general, contends the bill violates the supremacy clause of the U.S. Constitution that generally gives preference to federal laws over conflicting state ones and also infringes on First Amendment rights of free speech and press.

Koster, a Democrat, would be responsible for defending the measure against a lawsuit. He said Tuesday that a federal court may declare some provisions of the measure unconstitutional, such as the attempt to nullify federal gun-control laws. He said other parts likely would be upheld but could have negative consequences.

“While I will defend the portions of HB 436 that are defensible, my representation must clearly and emphatically distance my office from your proposed policy to prevent state law enforcement officers from 1) enforcing valid federal gun laws and 2) participating in joint law enforcement efforts,” Koster wrote to lawmakers.

Koster’s letter asks lawmakers to reread certain parts of bill and consider what could occur if it actually becomes law.

Rep. Doug Funderburk, who sponsored the bill, said Koster’s letter only increases his desire to override the veto.

“His focus is the same as the governor’s—it’s just to defeat the successes of the legislative session in a partisan political way,” said Funderburk, R-St. Peters.

House Speaker Tim Jones, R-Eureka, dismissed Koster’s letter as “politically motivated.” He said in a written statement that the bill “seeks to affirm our rights as a state by pushing back against a federal government that has far exceeded the authority it was intended to have by our founding fathers.”

Koster, who touted an endorsement from the National Rifle Association in his 2012 campaign, said Tuesday that he is “a fellow supporter of the Second Amendment. “

The NRA has not publicly taken a position on the Missouri legislation and has not responded to repeated messages from The Associated Press over the past couple months inquiring about whether the organization supports or opposes the bill.

Koster said the measure would make it illegal for a Missouri State Highway Patrol trooper to refer to federal prosecutors a felon caught selling guns to people living in the U.S. illegally—an action that he said isn’t addressed in Missouri law but is a crime under federal law.

Other parts of the measure cite specific federal laws to be nullified, such as a 1934 law that imposed a tax on transferring machine guns and silencers. The Missouri measure also would allow civil lawsuits against those who attempt to enforce any laws infringing on people’s Second Amendment rights.

Koster said a police officer who discovers a fully automatic machine gun in a drug dealer’s car could be sued by the drug dealer for attempting to enforce federal laws if the officer refers the matter to the U.S. attorney’s office.

Personnel in U.S. Attorney General Eric Holder’s office have not responded to Associated Press requests for comment about the Missouri measure.

Funderburk said Koster was reading too much into the legislation. He said a court first would have to determine that a federal law infringes on Second Amendment rights before it is considered invalid in Missouri.